Effect of Structural Heterogeneity in Chemical Composition on Online

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The Effect of Structural Heterogeneity in Chemical Composition on Online Single Particle Mass Spectrometry Analysis of Sea Spray Aerosol Particles Camille M Sultana, Douglas B. Collins, and Kimberly Ann Prather Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b06399 • Publication Date (Web): 16 Mar 2017 Downloaded from http://pubs.acs.org on March 22, 2017

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Environmental Science & Technology

The Effect of Structural Heterogeneity in Chemical Composition on Online Single Particle Mass Spectrometry Analysis of Sea Spray Aerosol Particles Camille M. Sultana1, Douglas B. Collins1†, Kimberly A. Prather1,2,* 1

Department of Chemistry and Biochemistry, University of California, San Diego, La

Jolla, CA 92093-0314; 2

Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA

92093; 1

ABSTRACT

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Knowledge of the surface composition of sea spray aerosols (SSA) is critical for

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understanding and predicting climate-relevant impacts. Offline microscopy and

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spectroscopy studies have shown dry supermicron SSA tend to be spatially

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heterogeneous particles with sodium and chloride rich cores surrounded by organic

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enriched surface layers containing minor inorganic seawater components such as

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magnesium and calcium. At the same time, single particle mass spectrometry reveals

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several different mass spectral ion patterns, suggesting that there may be a number of

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chemically distinct particle types. This study investigates factors controlling single

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particle mass spectra of nascent supermicron SSA. Depth profiling experiments

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conducted on SSA generated by a fritted bubbler and total ion intensity analysis of SSA

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generated by a marine aerosol reference tank were compared with observations of

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ambient SSA observed at two coastal locations. Analysis of SSA produced utilizing

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controlled laboratory methods reveals that single particle mass spectra with weak sodium

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ion signals can be produced by the desorption of the surface of typical dry SSA particles

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comprised of salt cores and organic rich coatings. Thus, this lab-based study for the first

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time unifies findings from offline and online measurements as well as lab and field

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studies of SSA particle mixing state.

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1. INTRODUCTION

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Atmospheric aerosols impact climate by interacting directly with incoming solar radiation

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and indirectly through influencing cloud properties by serving as cloud condensation

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(CCN) and ice nuclei.1,2 Sea spray aerosols (SSA) represent one of the most abundant

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types of tropospheric aerosols.3 The ability to determine the radiative forcing of

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anthropogenic aerosols is limited by the uncertainty of the impact of large natural sources

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of aerosols on current total aerosol radiative forcing.4 SSA are ejected into the

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atmosphere when bubbles burst at the air-sea interface.5 Previous field and lab studies

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have shown that SSA is a complex mixture of inorganic salts and an array of dissolved

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and particulate organic components.6,7 Modeling studies have indicated that the mixing

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state of submicron SSA can have important effects on predicted CCN concentrations,8–11

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while the three dimensional chemical structure and mixing state of supermicron SSA can

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affect light scattering due to changes in water uptake at sub-saturated relative

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humidities.12,13 Additionally, the supermicron size range is where the bulk of SSA surface

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area resides, and thus plays a key role in light scattering and interactions with gaseous

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species.14–16

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In an effort to characterize the diversity in chemical mixing state of SSA, field

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studies employing off-line microscopy and spectroscopy techniques have helped illustrate

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the array of submicron SSA particle types.7,17–21 In contrast, offline chemical analysis


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techniques have found the vast majority of dry supermicron particles to be phase-

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separated inorganic cuboids with amorphous coatings.7,19–32 Elemental analyses have

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shown that the inorganic cores are mainly composed of sodium chloride, while organic

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compounds along with minor inorganic species, such as magnesium, potassium, and

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calcium, are heavily enriched in the amorphous coating or surface localized nodules.19–26

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These results are in agreement with efflorescence studies of particles generated from

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natural seawater and model salt solutions, which demonstrated that after drying the

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particles consist of sodium chloride cores with the particle surface enriched in the minor

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inorganic components such as magnesium, potassium, and calcium.33–40 Although the

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amount of the coating material can vary between particles, offline chemical analysis has

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generally shown a single type of dry supermicron SSA particle with the population

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represented as an internal mixture of salt and organic species. While it is known that the

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bubble bursting process can eject both fragmented and intact microbiological cells,23,41

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findings from spectromicroscopy techniques suggest that particles containing whole cells

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are rare. However, the collection and analysis methods are often not optimal for

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preserving cellular structures, and the sample throughput is often low. Consistent with

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this result, single bubble bursting experiments have reported the fraction of collected film

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and jet drops containing bacteria cells to be ~1% and less than 0.1%, respectively.41–43

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However, analysis of dried SSA by online single particle mass spectrometry (SPMS) has

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described an externally mixed population. Analysis based upon mass spectral signatures

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has identified three distinct particle types contributing significantly to the supermicron

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SSA population: sea salt (SS), sea salt with organic carbon (SSOC), and biological or

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magnesium (Bio) particles.28,44–46 Bridging the data from both offline and online methods,


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as well as understanding the factors that affect the mixing state of supermicron SSA

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particles is a requirement for understanding the water uptake and light scattering

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properties of marine aerosols.

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Particles with core-shell morphologies, comprised of specific components making up the

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core of the particle with other chemical species forming an outer layer were generated in

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previous laboratory studies and studied with SPMS.47–51 By varying the energy of the

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laser pulse that desorbs and ionizes the core-shell particles, these studies have illustrated

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that depth profiling of particles can be accomplished in real time. Lower laser fluence

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tends to partially desorb the particle, generating mass spectra (MS) that are representative

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of the surface components, whereas higher laser fluence typically desorbs a greater

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fraction of the particle, generating MS with greater ion signal contribution from core

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components. However, it has also been shown that even performing laser

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desorption/ionization using a consistent laser pulse energy on well-controlled particles of

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similar composition, there can be considerable variation in the laser fluence experienced

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by individual particles resulting in a range of mass spectra.47,49,51,52 Considering the

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structural observations of SSA by spectromicroscopy, combined with the variations in

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desorption/ionization common to SPMS, the range of MS patterns generated by SSA may

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be, at least in part, a result of the structural heterogeneity of dry supermicron SSA. Total

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ion intensity of each mass spectrum has been utilized as an indirect measure of the extent

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of particle desorption (proportional to the laser fluence experienced by the particle), and

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on average, mass spectra of SSA particles that had low total ion intensity were relatively

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rich in Mg, K, and Ca.22


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This study extends the complexity of previous single particle mass spectrometry

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studies in an effort to inform the existing descriptions of the chemical mixing state of

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supermicron SSA particles. Aerosol time-of-flight mass spectrometry (ATOFMS) was

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used to perform a detailed analysis of laser desorption/ionization mass spectra of dry

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supermicron SSA sampled from three different sources: (1) SSA generated by bubbling

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seawater using a sintered frit, (2) SSA generated in a Marine Aerosol Reference Tank

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(MART), and (3) SSA sampled in ambient aerosol during two coastal field studies.

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Depth profiling of frit-generated SSA was performed using ATOFMS. The

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results from the depth profiling study, the first to be performed on SSA particles utilizing

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SPMS, are extended to the MART and ambient generation schemes by examining the

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relationship of the relative sodium ion signal to mass spectral patterns and total ion

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intensity. Finally, this new approach is applied to sodium deficient and sodium rich SSA

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mass spectra collected during two coastal field studies. This study aims to unify the

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description of the mixing state and chemical composition of supermicron SSA by offline

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spectroscopy and microscopy techniques with real-time analysis by single particle mass

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spectrometry.

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2. METHODS

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2.1 SSA generation

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2.1.1 Fritted Bubbler

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On 1/18/16 seawater was collected from the coastal Pacific Ocean at Scripps Pier

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(La Jolla, CA; 32°51´56.8"N: 117° 15´38.48"W; 275 m offshore) at least 5 meters below

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the low tide line and passed through a sand bed filter to remove large debris. The water

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was then filtered through 0.8 µm and then 0.2 µm filters to remove biological particles


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such as bacteria and phytoplankton cells. 350 mL of the filtered seawater was then added

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to a 500 mL gas washing bottle or ‘fritted bubbler’ (Ace Glass) previously combusted at

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400 °C for 6 h. Zero air (Sabio 1001) was flowed through the sintered glass filter or ‘frit’

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(Pore Size C: 25-50 µm), submerged at a depth of roughly 17 cm, at a rate of 0.04 L/min

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to generate bubbles that rose through the seawater column, breaking at the surface to

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generate SSA particles used in the analyses described below. Additional zero air was

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added to the sample line after the bubbler to maintain a flow rate above 1 LPM (~1.25

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LPM). Silica gel diffusion dryers were placed between the bubbler and the ATOFMS

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inlet to dry the particles (RH < 10%).

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2.1.2 MART

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Coastal Pacific seawater (60 L) was collected from the ocean surface at Scripps

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Pier (La Jolla, CA; 32°51´56.8"N: 117° 15´38.48"W; 275 m offshore) on 6/11/13 16:00

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and added without treatment or filtering to a 100 L MART system.53 At the time of

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collection, the chlorophyll-a concentration, water temperature, and salinity were 0.04

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mg/m3, 19.4 °C, and 33.6 PSU. SSA particles were generated in the MART using the

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pulsed plunging waterfall technique described in detail previously, with a 4 second

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waterfall duty cycle.53 Two silica gel diffusion dryers were placed between the MART

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and the ATOFMS inlet to dry the particles (RH ~15%).

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2.1.3 CIFEX

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ATOFMS measurements of ambient aerosols were made during the Cloud

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Indirect Forcing Experiment (CIFEX) at a coastal site in Trinidad Head, CA (41.05° N:

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124.15° W) in April 2004.54 Aerosols were collected at a height of 10 m, and the

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sampling inlet was heated to maintain the RH at 55%.


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2.1.3 CalWater2

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ATOFMS measurements of ambient aerosols were made during the CalWater2

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field study at a coastal site in Bodega Bay, CA on the grounds of Bodega Marine

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Laboratory (39.32° N: 123.07° W) from January to March 2015.55 Aerosols were

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collected at a height of five meters and silica gel diffusion dryers were placed before the

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ATOFMS inlet to reduce the RH of sampled air to ~15%.

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2.2 Measurement of SSA Composition via ATOFMS

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The size-resolved chemical composition of individual dry SSA particles with

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vacuum aerodynamic diameters (dva) between 1 - 3 µm were measured in real time by

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ATOFMS. The measured vacuum aerodynamic size distribution of the sampled particles

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is provided in Figure S1 along with estimated aerodynamic and volume equivalent

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distributions (Method S1). A detailed discussion of ATOFMS has been given

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previously,56,57 so only a brief description follows here. Aerosol particles are drawn into a

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differentially pumped vacuum chamber through a converging nozzle inlet, wherein

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particles are accelerated to their size-dependent terminal velocities. Aerosol particles are

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sized based on the time required to transit two continuous wave laser beams (532 nm).

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The velocity is converted to vacuum aerodynamic diameter via calibration with

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polystyrene latex spheres (Invitrogen) of known diameter and density. When each

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particle arrives in the ion source region of the mass spectrometer, a Q-switched Nd:YAG

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laser pulse (266 nm wavelength, 8 ns pulse width, 700 µm spot size), triggered based on

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the velocity of particle, desorbs and ionizes each particle’s chemical components. Laser

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pulse energy was kept constant at approximately 1.1-1.3 mJ for the MART, CIFEX, and

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CalWater2 studies. For the frit-generated SSA studies, the energy of the laser pulse was


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varied between 0.6-1.5 mJ, and mass spectra from 800-1300 particles were collected at

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each laser energy. Below 0.6 mJ, the percentage of particles generating MS dropped

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considerably, precluding the use of such low laser pulse energies for these experiments.

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The exact laser fluence that each particle experiences is variable even at a constant laser

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energy setting due to the Gaussian profile of the laser beam, hot spots in the laser beam,

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and shot-to-shot variations in laser pulse characteristics.52 The positive and negative ions

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produced are detected by a dual-polarity reflectron time-of-flight mass spectrometer.

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Single particle mass spectra and size data were analyzed using the software toolkit,

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FATES.58

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2.3 Analysis of Particle Mass Spectra

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Note that all analyses included in this study were performed on particles with dva

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between 1 and 3 µm. Particles were binned by the fraction of positive mass spectral

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intensity attributable to sodium and sodium chloride containing ions (FNa): 23Na+, 46Na2+,

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81,83

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correspond to the most likely ion produced at a specific mass-to-charge ratio (m/z). For

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consistency, only positive sodium ion markers were utilized as negative MS were not

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detected for every instance of a positive MS. Note that total positive ion intensities are

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the summation of all positive ion signal and are normalized to the maximum value within

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each dataset. Based upon similarity to the mass spectral fingerprints of the lab-generated

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SSA a subset of mass spectra most likely to represent freshly generated SSA particles

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were identified within the field study datasets. Mass spectra that contained indications of

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terrestrial or anthropogenic sources (e.g. nitrate, biomass burning, dust, soot) were

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eliminated. Mass spectra identified as likely generated by cellular material based upon

Na2Cl+, 139,141,143Na3Cl2+, 197,199,201Na4Cl3+, 255,257,259Na4Cl3+. Peak assignments


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comparison to the literature, with dominant potassium and phosphate ion markers,59–62

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were not included in the SSA analysis for the field studies due to the possibility of a

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biological terrestrial source. See supporting information for more detail on the analysis of

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the laboratory and field data.

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3. RESULTS AND DISCUSSION

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3.1 Effect of Laser Power on Supermicron SSA Mass Spectra

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Supermicron SSA particles (dry dva = 1–3 µm) generated from natural seawater

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utilizing a fritted bubbler were analyzed via ATOFMS, varying the pulse energy of the

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desorption/ionization laser from 0.6 to 1.5 mJ. It is important to note that the bubbled

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seawater was filtered to remove most insoluble biological components such as bacteria

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and phytoplankton cells and preclude such material from being aerosolized and

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contributing to the mass spectra generated. Data from all laser energy conditions were

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compiled, and then the average normalized MS for particles with differing fractions of

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positive mass spectral intensity from sodium were calculated. For the purposes of this

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study, all particles with FNa, fraction of sodium-containing positive ion signal, greater and

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less than 0.4 are referred to as Na-Rich and Na-Deficient, respectively. Mass spectra with

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a lower FNa have higher relative contributions from components that are expected to exist

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at the surface of dried SSA particles, including calcium (40Ca+, 57CaOH+, 75CaCl+

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96

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109,111,113

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59

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S2). Interestingly, the average normalized MS with high (0.8-1), medium (0.4-0.6), and

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low (0.0-0.1) FNa are similar to the representative mass spectral fingerprints of the SS,

Ca2O+, 145,147,149CaCl3-), magnesium (24Mg+, 129,131,133MgCl3-), potassium (39K+, KCl2-), and organic material (27C2H3+, 27CHN+, 37C3H+, 43C2H3O+, 43CHNO+,

C3H9N+, 26CN-, 42CNO-, 43C2H3O-) in both negative and positive mass spectra (Figure


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SSOC, and Bio type particles that have been identified by previous SPMS analyses.44,46,63

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Based solely on these results two potential explanations present themselves. Firstly the

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three types of mass spectra could be derived from a single population of phase-separated

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particles with chemical spatial heterogeneity, such as the core-shell morphology

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previously described. Alternatively a collection of three somewhat distinct particle types

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could exist in the aerosol population. Results from the different laser pulse energy trials

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were used to distinguish between these two possibilities by providing information on the

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three dimensional chemical morphology of the particles.

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Previous studies utilizing SPMSs have reduced the pulse energy of the

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desorption/ionization laser to selectively detect components on the surface of particles

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with known core-shell morphologies.47–51 As the laser pulse energy was increased from

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0.6 to 1.5 mJ the fraction of Na-Rich MS dramatically increased from 39% to 77.5%

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(Figure 1). As indicated in the schematic in Figure 1, the greater sodium signal at higher

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laser energy (more complete particle desorption/ionization) is consistent with structurally

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heterogeneous particles in which sodium is located in the core. Lower laser energies were

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more likely to desorb/ionize only small amounts of each particle, and the generated mass

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spectra had relatively greater contributions from components such as magnesium and

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organics. As discussed earlier, spectromicroscopy studies have shown that dried nascent

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SSA are dominated by a single type of structurally heterogeneous particles with

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magnesium, calcium, sulfate, potassium, and organics concentrated on the outside in an

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amorphous coating or sometimes in distinct nodules surrounding cores of sodium

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chloride.20–23 The depth profiling results indicate that the variety of supermicron SSA


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mass spectra observed by ATOMFS could be attributed to a population of core-shell

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particles encountering a range of desorption/ionization conditions within the instrument.

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3.2 Relationship of Chemical Signal to Total Positive Spectrum Ion Intensity

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Studies of lab-generated particles with well characterized and controlled core-

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shell morphologies have shown that total positive ion intensity increases with increasing

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relative contribution from components in the core of the particle.47,64 The use of total ion

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intensity as a method to discriminate between mass spectra that were generated in

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association with varying degrees of particle desorption (surface vs. core) has been

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employed previously for lab-generated SSA.22 Particles analyzed in this study, however,

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were grouped by the fraction of positive ion signal from sodium rather than by the

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explicit total ion intensity as in the prior study. The total positive ion intensity

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distribution for each group of mass spectra binned by FNa was calculated for all

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supermicron particles. The results for the 1.2 mJ laser energy trial are shown in Figure 2,

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and are representative of the results across all laser pulse energy (0.6-1.5 mJ) ranges

234

examined (Figure S3). At higher FNa the total positive ion intensity distribution shifted to

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higher values (Figure 2-3). The increase in total positive ion intensities with increasing

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relative sodium ion signal is consistent with increasing particle desorption as shown in

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the conceptual cartoon in Figure 1. These results are in good agreement with previous

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analyses of SSA particles by ATOFMS that grouped particles explicitly by total mass

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spectral intensities.22 However, even when sodium rich mass spectra were detected,

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particle desorption was likely not complete as the total positive ion MS intensity did not

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increase with size (Figure S4).


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The generally low positive ion intensity of Na-Deficient mass spectra supports the

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conclusion that these MS are mainly representative of the particle surface which has been

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shown to be enriched in organics and minor inorganic seawater components via offline

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techniques. However, it appears that the outlying sodium deficient mass spectra with

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relatively high total ion intensities also have high contributions from positive calcium ion

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markers (40Ca+, 57CaOH+, 75,77CaCl+, 96Ca2O+) (Figure 3). This behavior is likely still

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attributable to the morphology of dry SSA particles. For both SSA generated by the

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fritted bubbler and the MART the relative calcium ion contribution and the relative

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sulfate ion contribution (48SO-, 64SO2-, 80SO3-) have a positive correlation (r2bubbler = 0.35,

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r2MART = 0.44) (Figure S5). Calcium sulfate, calcium sulfate hydrates, and sodium

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calcium sulfate salts are well known to precipitate and crystallize initially during the

253

dehydration of seawater and seawater droplets.36,37,65 Nodules or crystals rich in calcium

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and sulfur have been observed in dry SSA particles adhered to the outside of sodium

255

chloride rich cores.20,22 It is possible that desorption and ionization of calcium sulfate

256

crystals on the surface of dry SSA particles generates sodium deficient mass spectra but

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with more ion intensity than when only desorption of the amorphous and relatively thin

258

coating occurs. The infrequency of these relatively intense calcium rich mass spectra

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may be because these crystals or nodules are less ubiquitous than the amorphous coating

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and localized to a specific region of a particle. Overall the relationship between the

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positive mass spectral ion markers and total positive ion intensity is consistent with the

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known core-shell morphologies of dry supermicron SSA.

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3.3 SSA Mass Spectral Consistency between Laboratory and Field Studies


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The data analyses described in the previous sections for SSA generated from

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bubbled seawater were also performed on dry supermicron SSA generated by a MART

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and from two field studies (CIFEX, CalWater2). For the MART and field studies, the

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particles were binned by FNa (Table S1) and the average mass spectra were similar to the

268

results from SSA generated by bubbled seawater (Figure S2). In addition, for all four

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datasets the population of mass spectra with the highest FNa (0.8-1) had higher total ion

270

intensities than the preponderance of mass spectra with the lowest FNa (0-0.1). These

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results strongly suggest that the supermicron SSA generated by the fritted bubbler,

272

MART, and sampled during field studies are all similar in composition and morphology.

273

Therefore, for all datasets the supermicron SSA population is likely dominated by a

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single particle type that, once dried, is structurally heterogeneous. Such particles exhibit a

275

sodium chloride core with a surface enriched in organic material and minor inorganic

276

seawater components. Some differences in the thickness of the sodium chloride deficient

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particle coating are likely to exist, as illustrated by images of typical ambient, MART,

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and frit generated dry SSA particles from prior studies (Figure S6).28 While the

279

morphology of dried SSA particles will not be representative of their hydrated structure

280

under relatively humid ambient conditions, studying dried phase-separated particles

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provides a standard and experimentally simple common basis for analyzing chemical

282

composition across an array of different methods.

283

Results from this study demonstrate that sodium deficient mass spectra, which are

284

relatively rich in species such as magnesium, can be generated from desorption and

285

ionization of the surface of dried SSA. Since the acquisition of mass spectra that

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differentially represent the core and surface coating of dry SSA is mostly a function of


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the instrumental operating conditions (e.g., laser pulse energy), the ratio of sodium rich to

288

sodium deficient mass spectra should be relatively constant. To examine this hypothesis,

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the fraction of identified fresh SSA Na-Rich and Na-Deficient mass spectra within two-

290

hour time bins was calculated for two field studies performed on the west coast of North

291

America, CIFEX and CalWater2 (Figure 4). In both studies the fraction of Na-Deficient

292

mass spectra to the fraction of Na-Rich mass spectra shows a generally positive trend as

293

expected, though the correlations are poor (r2CIFEX = 0.28, r2CW2 = 0.17). Notably, the

294

CIFEX results here are in close agreement with a previous analysis where the MS were

295

grouped by a clustering algorithm and “fresh SS” and “Mg-type” particle types, very

296

similar to the average MS for high and low FNa (Figure S2) respectively, were found to be

297

well correlated.63

298

To further support the assignment of the Na-Deficient mass spectra as fresh SSA,

299

versus terrestrial or anthropogenic particle types, the mean relative nitrate and sulfate ion

300

signal (46NO2-, 62NO3-, 97HSO4-, 125H(NO3)2-, 147Na(NO3)2-, 160HNO3HSO4-,

301

188

302

phase nitric and sulfuric acid is indicative of air masses influenced by anthropogenic

303

pollution and secondary aging processes. It is anticipated that the fraction of Na-

304

Deficient, fresh SSA mass spectra will be negatively correlated with air masses not

305

abundant in fresh SSA, and therefore the mean relative nitrate ion signal and sulfate ion

306

signal. The results show a general negative trend (r2CIFEX = 0.48, r2CW2 = 0.35).

(HNO3)2NO3-) for all mass spectra detected was calculated for each time bin. Particle

307

While the resulting relationships generally support the conclusion that the

308

identified Na-Rich and Na-Deficient mass spectra originate from the same particle type,

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large variation exists in the data. Variations in the Na-Rich to Na-Deficient ratio may be


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driven by the analytical difficulties associated with utilizing field study datasets for this

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specific type of investigation. This analysis depends upon accurately identifying SSA

312

mass spectra in an uncontrolled environment. However, there are a variety of particle

313

types, such as dust, biomass burning, and cellular material, which can generate mass

314

spectra rich in magnesium, calcium, potassium, and/or organic ion markers. It is possible

315

that some Na-Deficient mass spectra included were not generated by surface desorption

316

of SSA particles. Additionally it is key to include only MS from fresh SSA that have not

317

been aged by secondary processes which have been shown to change the three

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dimensional physicochemical structure of SSA.22 However, negative MS, which

319

frequently contain the secondary processing ion markers, are generated with less

320

frequency than positive MS, particularly for low ion intensity mass spectra.

321

In addition, changes in seawater chemistry have been shown to influence SSA

322

organic content and therefore also likely modify the thickness of the sodium deficient

323

shell.10,66,67 It is expected that increases in the thickness of the organic coating on dry

324

supermicron SSA particles would directly correspond to an increase in the fraction of Na-

325

Deficient mass spectra generated at a constant laser pulse energy.47,50 Therefore some of

326

the variation in the ratio of Na-Rich to Na-Deficient MS noted in the field studies may be

327

a result of changing seawater chemistry. While the paradigm that this study establishes

328

with respect to interpretation of single particle mass spectra of SSA should find facile

329

utility for studies of nascent SSA, there are still difficulties in applying this framework to

330

complex field study datasets.

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3.4 Contribution of Particulate Biological Components to SSA MS


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The MART and bubbled seawater datasets were highly similar, however only the

333

bubbled seawater was filtered to remove microbial cells. This suggests that intact cells or

334

cell fragments were not abundant in the MART supermicron SSA or that mass spectra

335

generated by aerosolized cells could not be reliably distinguished from those obtained

336

from other salt and organic matter-containing supermicron SSA particles. As described,

337

ion signatures which have been previously associated with cellular single particle mass

338

spectra, such as organic, potassium, calcium, and phosphate ion markers, can be

339

generated from the surface desorption of the structurally heterogeneous dried

340

supermicron particles. It is important to note that while intact biological particles can be

341

ejected in and have been identified in SSA,23,68 it is difficult to unequivocally assign a

342

mass spectral pattern to aerosolized marine cells without more subtle data analysis

343

methods or further information about the particles’ properties. Therefore, further

344

correlated information (e.g., fluorescence) may be required to identify and quantify the

345

abundance of microbial cells ejected from the surface ocean.

346

3.5 Interpreting mass spectra from SPMS studies of SSA

347

Other than ejected particles composed of particulate biological components, the

348

chemical composition of most supermicron SSA in the size regime examined (dva = 1-3

349

µm) is expected to be relatively similar between particles.7 However within each particle,

350

spectromicroscopy analyses have shown a great deal of spatial heterogeneity in the

351

structure of dry SSA particles.19–24,26,69 Studies of dehydrated natural and model seawater

352

droplets show that particles exhibit a core-shell morphology, with sodium chloride rich

353

cores and nodules of calcium sulfate and an amorphous coating rich in magnesium,

354

potassium, organics and other minor inorganic components at the surface.33–40 SPMS


16

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355

analysis of morphologically-controlled core-shell particles has shown that even at a

356

constant laser pulse energy setting there is wide particle-to-particle variability in the ratio

357

of signal from surface and core components in single particle mass spectra.47,49,51 This

358

variation exists despite the chemical and morphological similarities between the lab-

359

generated core-shell particles. As seen in this study, mass spectra with a wide range of

360

sodium fractions were generated for all laser energies, which is consistent with previous

361

studies of core-shell particles. However, the wide variability in signal response illustrates

362

the complexity in analyzing SPMS data sets.

363

The schematic in Figure 5 illustrates that when analyzing single particle mass

364

spectra of SSA, or any population of particles with structural heterogeneity, distinctions

365

between relative mass spectra are not necessarily indicative of an externally mixed,

366

chemically distinct particle population. Rather we have shown that varying degrees of

367

laser desorption and ionization of core-shell SSA particles can produce MS with high,

368

medium, and low sodium ion contributions which have been previously characterized as

369

originating from distinct SS, SSOC, and Bio SSA particle types respectively.28,44–46

370

Utilizing the paradigm established, increases in the fraction of low intensity and sodium

371

deficient mass spectra generated by dried supermicron SSA should likely be interpreted

372

as increases in the thickness of the amorphous coating on typical SSA particles rather

373

than an appearance of a new distinct sodium deficient particle type. Leveraging depth

374

profiling analysis and examining trends in total ion intensity coupled with known

375

morphology from spectroscopy and microscopy studies has helped enable this distinction.

376

These results bring real-time chemical analysis of SSA by SPMS into agreement with

377

offline microscopy and spectroscopy descriptions of the supermicron SSA population,


17


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378

which dictates SSA’s direct light scattering properties and interactions with gaseous

379

species.14–16 Importantly, this analysis and new understanding can serve as a foundation

380

for SPMS analysis of SSA, which could be extended in the future to the more complex

381

submicron SSA population. It is important to note that the explicitly varying the laser

382

pulse energy proved to be a useful tool for understanding the three-dimensional structural

383

morphology and composition of atmospheric SSA particles.

384

ASSOCIATED CONTENT

385

Supporting Information Method for converting particle diameter; Experimental method

386

for generating and imaging SSA particles in Figure S6; A table providing the number of

387

mass spectra in each sodium fraction bin for all experiments; Size distribution for

388

particles detected and analyzed by the ATOFMS; Average relative mass spectra are

389

shown for supermicron SSA and grouped by the fraction of positive mass spectra

390

intensity from sodium; The total positive ion spectrum intensity versus size for the

391

bubbled seawater experiment; The total ion intensity distribution for all bubbled seawater

392

data; The calcium ion fraction plotted against the sulfate ion fraction for individual mass

393

spectra; Compilation of images of SSA particles.

394

AUTHOR INFORMATION

395

Corresponding Author

396

*E-mail: [email protected]. Phone: +1 858 822 5312. Address: Department of

397

Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA 92093-

398

0314.

399

Present Addresses

400

† Department of Chemistry, University of Toronto, Toronto, ON, Canada M5S 3H6


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401

ACKNOWLEDGMENTS

402

This work was supported by the National Science Foundation through the Centers of

403

Chemical Innovation Program via the Center for Aerosol Impacts on Climate and the

404

Environment (CHE-1305427).

405

supplemental atomic force microscopy analysis, all collaborators involved in the MART

406

microcosm studies, notably C. Lee, as well as J. Holecek and G. Cornwell for field study

407

data collection. In addition the authors would like to acknowledge the Bodega Marine

408

Reserve, University of California Davis, and UC Natural Reserve System for use of

409

Bodega Marine Laboratory facilities.

410

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Figure 1

Increasing relative signal from core components

Figure 1. The distribution of the relative sodium ion contribution to the total positive ion intensity for supermicron SSA, generated by frit bubbled seawater analyzed by ATOFMS using five different desorption/ionization laser pulse energies.


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Figure 2

Figure 2. The distribution of total positive ion intensity grouped by the fraction of positive mass spectral sodium intensity for supermicron SSA particles generated by the (a) frit bubbled seawater and (b) MART, and sampled during (c) CIFEX and (d) CalWater-2. Total intensity values were normalized within each dataset.


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Figure 3

Figure 3. The sodium ion fraction versus the relative total ion intensity for each positive mass spectrum generated by supermicron SSA from the (a) frit bubbled seawater and (b) MART (1.2 mJ laser pulse energy). Warmer marker shades indicate higher calcium ion fraction in the positive mass spectra.


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Figure 4

Figure 4. (a) The temporal trends for the fraction of ambient particles identified as SSA with sodium fractions of 0.4-1 (Na-Rich) and 0-0.4 (Na-Deficient) shown for two coastal field studies, CalWater2 and CIFEX. (b) Scatter plots of the number fraction of NaDeficient particles against the number fraction of Na-Rich particles and (c) the relative mean signal from nitrate and sulfate markers in the total aerosol population.


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Figure 5

Figure 5. A schematic illustrating how two different particle populations could both generate a similar suite of relative mass spectra by laser desorption/ionization, the ionization method utilized by SPMS.


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TOC Graphic


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